The present invention relates to an inspection technology for comparing between a detected image and a reference image of an inspection target, which is obtained through the use, for instance, of light or laser beam, and detecting microscopic pattern defects, foreign matter, and the like based on the differences, and more particularly to a pattern defect inspection method and apparatus that are suitable for conducting a visual inspection of semiconductor wafers, TFTs, photomasks, and the like.
A known conventional technology for pattern defect inspection is disclosed, for instance, by Japanese Patent Laid-open No. 2001-5961.
A microscopic defect inspection apparatus is disclosed by Japanese Patent Laid-open No. 2001-5961. This microscopic defect inspection apparatus includes an image signal detection section, an analog-to-digital conversion section, a delay circuit section, a first image processing circuit section, a second image processing circuit section, and a defect judgment section. The image signal detection section, which includes radiation optical system for radiating DUV light having a wavelength of not more than 400 nm, and detection optical system equipped with a TDI image sensor or other similar image sensor, detects image signals having a pixel size of not larger than 0.2 μm from an inspection target and outputs the detected image signals in a parallel manner over a plurality of channels. The analog-to-digital conversion section forms each of the multi-channel image signals that are input in a parallel manner from the image signal detection section. The delay circuit section outputs multi-channel reference image data in a parallel manner. The first image processing circuit section performs a multi-channel, parallel image process to detect a positional displacement between the two types of image data and correct any positional displacement in accordance with the multi-channel detected image data derived from the analog-to-digital conversion section and the multi-channel reference image data derived from the delay circuit section. The second image processing circuit section performs a multi-channel, parallel image comparison process to compare reference image data and detected image data, which is received from the first image processing circuit section and subjected to positional displacement correction on an individual channel basis, and extract information about defect candidate points. The defect judgment section makes a detailed analysis in accordance with the information about multi-channel defect candidate points, which is input from the second image processing circuit section, and judges whether the defect candidate points are true defects.
Circuit patterns formed on semiconductor wafers targeted for inspection have been overly enhanced to a microscopic size of 0.1 μm or smaller while the diameters of semiconductor wafers have been increased. Since the circuit patterns are overly enhanced to a microscopic size, it is demanded that defects, which are smaller than circuit patterns in size, be detected. To fill such a demand, UV light or DUV light is used as the illumination light, and high-magnification detection optical system is used to achieve high resolution and provide a pixel size smaller than the defects to be detected. Consequently, the image information obtained from an inspection target will become huge. Under these circumstances, it is demanded that the obtained image information be rapidly processed to inspect for microscopic defects with high sensitivity and high reliability. However, these requirements are not adequately considered by the aforementioned conventional technology.
Due to the use of a CMP or other polishing method, semiconductor wafers targeted for inspection slightly vary in pattern film thickness. Therefore, local brightness differences are found in the image signals between various chips, which should basically be the same. In FIG. 4A, the reference numeral 41 denotes a typical inspection target image signal. In FIG. 4B, the reference numeral 42 denotes a typical reference image signal. As indicated by 4a in FIGS. 4A and 4b in FIG. 4B, the same patterns of the inspection target image signal and reference image signal differ in brightness. Further, there is an ultramicroscopic defect 4d in 41 in FIG. 4A, which is an inspection target image. In such an instance, the resulting difference image looks like FIG. 4C. The difference image is obtained by generating density differences in accordance with the difference values derived from various locations of the inspection target image and reference image. FIG. 4D shows a waveform that represents difference values derived from locations 1D-1D′. If any difference value exceeding a threshold value TH is labeled as a defect as is the case with the use of a conventional method, the difference value 4c, which represents the difference between patterns 4a and 4b, which differ in brightness, is detected as a defect. This is a problem that a false-information that should not be detected originally as a defect will occur so much. Such a false-information could be avoided, for instance, by increasing the threshold value TH (increasing from TH to TH2 as shown in FIG. 4D). However, the use of such a false-information avoidance method decreases the sensitivity so that the ultramicroscopic defect 4d, which corresponds to a difference value smaller than the increased threshold value, cannot be detected.